ASTM D4624 Standard Test Method for Measuring Apparent Viscosity by Capillary Viscometer at High-Temperature and High-Shear Rates
8. Calibration and Standardization
8.1 Calibration:
8.1.1 Constant Pressure Viscometer - Using a minimum of five Newtonian calibration oils covering the viscosity range from about 2 mPa•s (cP) to about 7 mPa•s (cP) at 150°C accurately determine viscosity as a function of flow rate (or flow time for a specified volume) for each pressure drop that is run.
NOTE 1 - When data are subsequently obtained with test oils, several values of pressure drop are needed to allow interpolation to 10(6)s(-1) shear rate. Fixing a regular schedule of pressure drops for this purpose is convenient and avoids the need to determine calibrations for every test run.
8.1.1.1 The calibration data can be presented either graphically, as plots of viscosity versus flow rate (or flow time) or mathematically. In either case, each pressure drop will have its own calibration curve. The following empirical relationship has been found to adequately describe the calibration data.
where:
V = viscosity, mPa•s (cP),
T = flow time for 20 mL, s (20 mL is the flow volume that is timed), and
a0, a1, a2, a3 = constants.
8.1.1.2 The following relationship, also, has been used successfully with capillary viscometers:
V = c1tP - c2/t
where:
V = viscosity, mPa•s,
t = flow time, for a calibrated volume, s,
P = pressure, kPa (1 psi 5 6.89 kPa), and
c1, c2 = constants.
8.1.1.3 Constant c2 will be essentially constant over the relatively narrow range of shear rates and viscosities of interest in measurement of the high-temperature viscosity of automotive engine oils. In more general applications, c2 may not be constant for all values of Reynolds Number.
8.1.2 Constant Flow Rate Viscometer - Using a minimum of five calibration oils covering the viscosity range from about 2 mPa•s (cP) to about 7 mPa•s (cP) at 150°C, accurately determine viscosity as a function of pressure for each flow rate that is run.
8.1.2.1 The calibration data can be presented either graphically, as plots of viscosity versus pressure, or mathematically. In either case, each flow rate will have its own calibration curve. The following empirical relationship has been found to adequately describe the calibration data:
V = b0 + b1P
where:
V = viscosity, mPa•s,
P = pressure, kPa, and
b0, b1 = constants.
8.1.3 Stability of Viscosity Calibration - Check the stability of the calibration by running a calibration oil in the same manner a test oil would be run. This is done no less frequently than every twentieth run. The calibration oil viscosity determined in this way must not differ from the standard value by more than the repeatability of the test (see 12.1). If it does, and if the result is confirmed by a repeat run, look for the source of the trouble, rectify it, and recalibrate if necessary. Some possible steps to find the source of the trouble are to check the system thoroughly for faults, verify the operating procedure, and check the temperature control and readout.
8.1.4 Stability of Temperature Calibration - Check the calibration of the temperature sensor at least once a year.
8.2 Standardization - Standardize each viscometer to verify its accuracy before its initial use and at least once a year thereafter. Standardization is accomplished by using the viscometer to measure the viscosity of a standard non-Newtonian reference oil. The measured viscosity must not differ from the certified viscosity of the standard by more than the reproducibility of the test (see 12.1). A standard reference oil for this purpose is commercially available.